There are many reasons for believing high chloride {100} tabular grain emulsions, the invention of Maskasky U.S. Pat. Nos. 5,264,337 and 5,292,632, to be ideal for a variety of photographic applications. Tabular grain emulsions are well known to offer improved sharpness and an improved speed-granularity relationship. Silver chloride emulsions are recognized to be ecologically attractive and to possess the capability of rapid processing. Silver chloride grains with predominantly {100} crystal faces are recognized to have a high degree of shape stability, allowing morphologically stable {100} tabular grains to be formed.
House et al U.S. Pat. No. 5,320,938 disclosed a process of preparing high chloride {100} tabular grain emulsions that employs iodide during grain nucleation. Subsequently Chang et al U.S. Pat. No. 5,413,904 disclosed an improvement in the House et al process resulting from delaying iodide addition in the nucleation step for a short period after initial silver and chloride ion addition.
Japanese origin patent disclosures (e.g., Saitou European patent applications 0 645 670 and 0 670 515 as well as Yamashita et al U.S. Pat. No. 5,641,620 and Oyamada et al U.S. Pat. No. 5,665,530) modified the Chang et al process by purporting to substitute "halide" for iodide in grain nucleation, although no "halide" other than bromide is demonstrated. These processes for preparing high chloride {100} tabular grain emulsions rely on an AgCl/AgBr/AgCl precipitation sequence (described as creating a "halide gap") during grain nucleation to create the crystal lattice dislocations that promote {100} tabular grain growth. After grain nuclei formation further precipitation results in the growth of high chloride {100} tabular grains.